Such tensioning and damping elements are elastic elements, which can be deformed between the shape of a concentric ring and the shape of a Cassinian curve with a shape close to an ellipse with an external toothed rim, capable of gripping the chain, and are disposed between the tight side and the slack side of the chain. As a result of the elliptical configuration of the Cassinian curve, geometric concepts, customary for an ellipse, are used in the further text.
The tensioning function results from the fact that chain elongations are equalized in that the tensioning and damping element, pre-tensioned in an elliptical shape, strives for a stressfree concentric shape between the tight and slack sides of the chain.
From the EP 0 625 653 B1, a tensioning and damping element is known, which is disposed between the tight side and the slack side of the chain and which, as an elastically deformable element, has a toothed rim with teeth, which are elastically deformable about the stress-free starting position, laterally open, arc shaped and, with the ring profiles in the root circle, pass into one another.
Furthermore, the EP 1 056 964 B1 and the U.S. Pat. No. 6,488,602 B1 disclose a tensioning and damping element, for which especially the ring part, as the elastic bending increases, shows a decreasing resistance to bending, which is cross section-dependent. The tensioning and damping elements correspond in shape to that of the chain, which they are capable of gripping.
The toothed rim thereof consists of elastic teeth, which are laterally open, have the shape of an arc and the roller-seatings of which pass into one another with the ring profile in the root circle. For the purpose of guidance clearance, the widths of the roller-seatings are less than the internal width of the associated chain.
In the operating state, there are different factors, which relate to the shape correspondence between the chain to the tensioning and damping element, so that, during the rotation of the tensioning and damping element, there is a periodically deformed, indifferent tooth profile with periodically deformed, indifferent pitch over the whole of the external extent.
Since toothed rim lies outside of the neutral bending line of the ring part, the pitch of the latter is compressed for the elliptical shape and smaller than for the concentric shape of the ring part.
For the end positions of the stress-free concentric shape and the pre-stressed elliptical shape of tensioning and damping element, there is, accordingly, no agreement between the pitch of the concentric shape, corresponding at the same time to the finished state, and the pitch of the toothed rim, with the pre-stressed elliptical shape, with the disadvantage of the compression of the toothed rim in the region of contact with the chain.
Since the pitch of the chain, which is not worn out at the beginning of the operating period, is smaller than the pitch of the chain, which is worn out at the end of the operating period, there are constantly varying pitch deviations during the operating time.
However, the regions of the chain engagement with the external teeth and roller-seatings are particularly important for the precise functioning of the tensioning and damping element, especially for the shape correspondence between the toothed rim and the chain.
The greater the sum of the tight side and the slack side of the chain in contact with the roller-seatings, the more stable is the position and the precision of the runout and the effectiveness of the tensioning and damping element.
With respect to the chain inlet and the chain engagement, all contacted roller-seatings, since they lie outside of the neutral bending fiber of the ring part, are connected in the case of an elliptical deformation of the tensioning and damping element in the regions in question with a type of closing movement of the roller-seatings and, accordingly also, with a temporary reduction in size of the pitch.
In the area of the chain engagement about the secondary vortexes of the elliptical shape, there is, especially when there is maximum deformation as at the start of the operating time with a new chain and, with that, a chain, which has not been elongated, compression of the roller-seating areas near the ground, with the result of an elastic camber of the roller-seating base of the ring part in the center of the roller-seatings. This leads to a point contact of the cylindrical rollers of the chain on the camber. Without contact between the chain rollers and the rounded free surfaces of the ring profile, the result is an unstable, oscillating runout of the tensioning and damping element between the inner link plates, visible as flutter between the tight and slack side of the chain.
This decreases the precision of the chain engagement and, with that, decreases in the security of the position and function of the tensioning and damping element in the orbit of the chain drive, with the result that the wear of the latter is increased.
The tooth profile of the known tensioning and damping element has arched, open teeth, which, in conjunction with a closing movement of the respective roller-seating, lead to the clamping contact with the chain roller, before the latter contacts the roller-seating base and, at the same time, forms a gap to the roller-seating base. By means of the mechanism of this closing movement with the formation of the gap to the roller-seating base, only the chain roller, which is in direct engagement and currently the main one in engagement, is contacted completely. In contrast with the chain with a regular, constant pitch, the elastically deformed roller-seatings, upstream and downstream from the contacted roller-seating, with a temporary smaller pitch dimension due to the deformation, are not in a precise engagement position of the chain with the regularly constant pitch of the latter. With that, there is a decrease in the wrapping contact by the chain.
As a result of this difference of setting-in clamping contact of the closing roller-seating with the chain roller, there is, however, during the compression and rebounding of the tensioning and damping element, an imprecise chain engagement between the regular pitch chain and the roller-seatings upstream and downstream from the contacted roller-seating of different deformation and pitch and, with that, differently increased running noise of the chain transmission.
A further changed factor of the form correspondence between the chain and the tooth profile of the tensioning and damping element, drifting from the initial operating state into the final operating state, consists therein that a drifting enlargement of the chain pitch goes along with the elongation of the chain from the running-in time and the wear period and at the operating temperature of the chain drive.
The greater the deformation during the installation in the chain drive, the stronger the roller-seatings are temporarily, disadvantageously made smaller in the engagement segment to the chain, since these lie outside of the neutral bending fiber of the ring part. During this diminution, they assume the profile of a stationary semi-elliptical curve, the main apex axis of which is orthogonal to the pitch circle. At the same time, in the toothed rim of the pre-tensioned tensioning and damping element, lying radially outside of the neutral bending fiber, there are regions with a compressed pitch in the area, in which there is contact with the chain.
With regard to the corresponding shape of the toothed rim and the chain, there accordingly are deviations in the precision in the case of an elliptical pre-tensioned, a partly pre-tensioned and a not pre-tensioned concentric tensioning and damping element as well as in the case of a not extended chain, a partly extended chain and a chain extended to the limit. Increasing and decreasing moments as well as the change between the tight side and the slack side of the chain during the reversing, produce changing contact angles as well as transitional deformations of the toothed rim and of the chain engagement and, with that, further precision deficiencies.
To summarize, during the operating period of the chain transmission, there are nonstationary engagement relationships between the toothed rim and the chain with regard to an imprecise chain engagement, which results from a large number of factors.
Due to the toothed rim, considered stationary according to the usual construction principles for chain sprockets, and configured concentrically, the latter, under operating conditions with the elliptical deformation of the tensioning and damping element, has unequal roller-seatings.
If these are suitable for the acting chain contact, they are compressed and made smaller for the commencing contact and stretched and enlarged for the loosening contact. These deviations, which, however, occur as errors only under operating conditions, increase with increasing chain speed and increasing compression.
All told, the known tensioning and damping elements, in conjunction with the wear of the chain, the load changing phases as well as the reversing according to the degree of deformation, have the disadvantage of a phased running irregularity, an increased running noise caused by engagement surges and of wear-intensive areas at the flanks of the teeth.
During their elliptical pre-tensioned state, the technical cause of these disadvantages of the known tensioning and damping elements consist of the unstable point contact of the chain rollers with the deformation-produced cambers of the ring-part profile in the vicinity of the root circle of the roller-seatings, in the differences of pitch and of shape correspondence with the chain as contact commences, in the acting contact and in the detaching contact of the dimension-determining chain contact of the engaging roller-seating with the upstream and downstream chain rollers, in the difference between the regular pitch of the toothed rim in the manufactured, concentric condition and the deformation caused reduced pitch, in the roller-seating closing in the elliptical pre-tensioned state as well as in the indifferent variance of the pitch of the toothed rim in the case of the different deformations in the operating period from the wear-dependent drifting enlargement of the pitch of the chain until the latter is worn out. For the tensioning and damping elements of the type described, the pitch of the toothed rim varies, depending on the function according to the degree of the elastic deformation of the elliptical shape.
Moreover, for the construction of concentric chain sprockets, the DE 102 37 277 A1 as well as the DE 200 06 278 U1 are known, for which the distance between pitches of the drive wheel or the chain sprocket, for the purpose of equalizing expansions, which depend on load and heat, as well as wear of the transmission or the chain, is greater than the distance between pitches of the counter-engagement element of the transmission or of the chain.
This constant enlargement of the pitch on the concentric pitch circle is related to the fixed chain sprockets and the wear up to the wear limit of the drive.
The elastic compression and expansion of the ring part, the deformation of the roller-seating, the deformation of the elastic toothed rim and the therewith associated omission of the pitch circle for the elastically deformed tensioning and damping element are not taken into consideration in an incomplete manner. The technical reason for this is the non-recognition of a Cassinian, elliptic curve of the pitch as a temporary restriction, which is additionally present in the case of an elastically deformed tensioning and damping element as an operating condition.